1. Technical Field
The present invention relates to a frequency measuring apparatus or the like, and in particular to improvement of a frequency measuring apparatus for making it possible to measure an absolute frequency using a frequency measuring apparatus for counting an input signal in a predetermined time period, and removing a high-frequency component from a counted value train, thereby detecting a frequency variation component.
2. Related Art
As a method of measuring a frequency, there are known a direct count method (see, e.g., JP-A-2001-119291) of counting pulses passing therethrough in a predetermined gate time, a reciprocal method (see, e.g., JP-A-5-172861) of accurately measuring the pulse period and obtaining the frequency using the inverse of the period thus measured, and a method (see, e.g., U.S. Pat. No. 7,230,458) of obtaining a ΔΣ modulation signal to thereby find out the frequency.
The applicant of the present patent application performs development of a frequency measuring apparatus using a short gate time count method as a novel proposal in addition to the methods described above.
The frequency count method described above is arranged to repeatedly count (sample) a signal including the pulse signal as the input signal in a short gate time without interruption, and then remove the high-frequency component from the train of the count values thus obtained to thereby extract the frequency variation component (a filtering process), which can significantly improve both of the time resolution and the frequency resolution. The frequency counter of the present method is composed of a counter circuit and a small-scale arithmetic circuit, and has an advantage of easily achieving a multi-channel configuration while suppressing growth of the circuit scale. Further, it also has a characteristic that the higher the sampling frequency (the inverse of the gate time of the counter circuit) is set, the more the resolution is improved.
In order for removing the high-frequency component described above, there is used a low pass filter. For example, in the case of using a digital filter as a configuration of the low pass filter section, a memory and an arithmetic device become necessary. By using a moving average filter as the low pass filter, an amount of operation can significantly be reduced, and the real-time measurement with high accuracy can be achieved. The counter circuit (a short gate time counter described later) has a simple configuration also as hardware, and is therefore suitable for a high-speed operation on the one hand, on the other hand the process in the filter section requires multi-bit addition and subtraction. Therefore, the upper limit of the sampling frequency in the real-time measurement is mainly determined by processing capacity of the filter section. In particular, when using the moving average filter of two or more stages, processing of multi-bit information becomes necessary, and therefore, the circuit thereof becomes complicated. If the circuit of the part described above is simplified, operation with a higher speed, a real-time processing, and reduction of power consumption become possible.